What is the lift force on a car's roof at 100km/hr with an area of 3.9m^2?

[salpal243]

the question I am stumped on is "What is the lift force on the roof of your car that has an aera of 3.9m^2 if you are driving 100km/hr? use 1.17kg/m^3 for density of air

 

[rcgldr]

There would have to be something deflecting air away from a flat roof in order to produce lift. Speed alone isn't going to create lift on a flat plate parallel to the relative wind. Bernoulli principle doesn't apply here. You could place a flush mounted static port in a flat roof to measure pressure of the moving air outside and it would indicate the same ambient pressure if the car was stopped or moving (as long as speeds are reasonably sub-sonic).

For a real car, the windshield deflects the air away from the roof, and the overall shape of most cars is similar to a common wing and tends to produce some lift. You'd have to know the profile shape of the car, similar to knowing the profile of an airfoil, in order to determine the lift versus speed.

 

[Lsos]

Speed alone isn't going to create lift on a flat plate parallel to the relative wind. Bernoulli principle doesn't apply here. You could place a flush mounted static port in a flat roof to measure pressure of the moving air outside and it would indicate the same ambient pressure if the car was stopped or moving (as long as speeds are reasonably sub-sonic).

Why does Bernoulli principle not apply? My first thought was that it would indeed create a significant difference in pressure...

 

[rcgldr]

Speed alone isn't going to create lift on a flat plate parallel to the relative wind.

Why does Bernoulli principle not apply?

Because the speed of the air wasn't the result of a transition from higher pressure to lower pressure. In this case, the assumption is that the pressure of the air is ambient. It doesn't matter what the speed of the flat plate is if the flat plate isn't interacting with the air (assume the flat plate doesn't change the speed of the air, ignoring any skin friction effects). This why static ports that are just small openings in the side of an aircraft's fuselage (at a point where the air's speed isn't being changed) can sense the ambient pressure of the air, regardless of the speed of the aircraft (as long as speed is sufficiently below the speed of sound).